What are CLD and STD for in x86 assembly language? What does DF do?

Question

well, I know that CLD clears direction flag and STD sets direction flag. but what\'s the point in setting and clearing direction flag?

Answer 1

CLD: Clears the DF flag in the EFLAGS register. When the DF flag is set to 0, string operations increment the index registers (ESI and/or EDI).

here is a simple example:

section .text
global main
main:
    mov ecx, len
    mov esi, s1
    mov edi, s2

    cld       ; redundant because DF is already guaranteed to be 0 on function entry
              ; but included for illustration purposes

loop_here:
    lodsb                ; AL=[esi],  ESI+=1 (because DF=0, otherwise ESI-=1)
    add al, 02
    stosb                ; [edi]=AL,  EDI+=1 (because DF=0, otherwise EDI-=1)
    loop loop_here       ; like dec ecx / jnz but without setting flags
    ; ECX=0, EDI and ESI pointing to the end of their buffers

    mov edx, len-1       ;message length, not including the terminating 0 byte
    mov ecx,s2           ;message to write
    mov ebx,1            ;file descriptor (stdout)
    mov eax,4            ;system call number (sys_write)
    int 0x80             ;call kernel

    mov  eax,1           ;system call number (sys_exit)
    xor  ebx,ebx
    int  0x80            ;call kernel: sys_exit(0)

section .data
s1: db 'password', 0        ; source buffer
len equ $-s1

section .bss
s2: resb len                ; destination buffer

(assemble and link with nasm -felf32 caesar.asm && gcc -no-pie -m32 caesar.o -o caesar. Or link it into a static executable with this as _start instead of main if you like.)

(this example tried to implement Caesar cipher.)